Water has been deteriorating by pollution as industries have developed. Water in rivers or lakes have gradually been polluted by waste material, especially toxic metals or chemicals, coming from chemical or metal treating factories. Although tap water used by common people is purified by several process, such as settling, filtering, disinfecting, chlorinating, and the like, it may still contain micro bacteria and invisible miscellaneous tiny impurities, so that it is unsuitable for drinking without boiling. Additionally, bacteria, miscellaneous impurities and chlorides may be consumed with raw vegetables which have been washed in the contaminated tap water. When rice is cooked in a rice cooker, the chloride contained in the tap water mixed in with the rice can become chloroform, which is toxic and harmful to a human body. The chloroform can mix in the rice cooked in the water and is consumed together with the rice.
Tap water is considered to have the following disadvantages:
1. It still contains considerable amounts of micro bacteria and miscellaneous impurities, not suitable for washing raw vegetables or fruit.
2. It contains some chlorides, so it can detrimentally affect the health of people who eat rice cooked with it.